Intraalloy Electron Transfer and Catalyst Performance: A Spectroscopic and Electrochemical Study

The synergistic promotion of three important fuel cell reactions on Pt-Ru electrodes suggests that electron transfer from ruthenium to platinum within the alloy may be responsible. Spectroscopic evidence for such a charge transfer is presented, and its consequences are analyzed not only for three fuel cell reactions, oxygen reduction and hydrogen and methanol oxidation, but also for two electrolytic reactions, hydrogen and oxygen evolution. It is pointed out that although a modest electron transfer in an alloy enhances the reactions synergistically, too large a charge transfer binds surface species adsorbed from the electrolyte too tightly to allow the reactants access to the electrode surface. This latter phenomenon is demonstrated on a Ni-Ti electrode, which is inactive for all three fuel cell reactions. However, Ni-Ti is stable in acid and gives normal electrolytic reactions, but with onset potentials and exchange-current densities lowered or raised in predictable directions. Chronopotentiograms for oxygen evolution and methanol oxidation reactions on 75-25 Pt-Ru electrodes are interpreted; they support the prediction that charge transfer enhances the adsorption of oxygen species from the electrolyte to the surface, which is the rate-limiting step in methanol oxidation.